1. Field of the Invention
This invention is directed to a method and apparatus for rapidly thermally processing a semiconductor wafer using electromagnetic radiation with a main irradiation arrangement which comprises at least one main irradiation source which transmits the majority part of the energy required for heating the semiconductor wafer and may have one or more secondary radiation sources and/or reflectors.
2. Description of Related Art
Rapid thermal processing methods are gaining in importance in the manufacture of electronic components on semiconductor wafers. The semiconductor wafers are individually introduced into a process chamber and are then very quickly and, insofar as possible, uniformly heated in a defined atmosphere, for example, by irradiation from an intense light source. The temperature is generally measured with a pyrometer at a measuring point in the center of the wafer. The signal from the pyrometer is to control the power to the heating lamps to control the intensity of the light so that a prescribed temperature-time cycle is produced with optimum precision. A typical temperature-time cycle for curing ion implantations is, for example, a heating rate of 300.degree. C. per second up to 1100.degree. C., a steady stage duration of 105, and then a cooling rate of 100.degree. C. per second.
A prerequisite for achieving high yields in rapid thermal processing is to maintain an adequately uniform temperature distribution over the entire semiconductor wafer. Particularly with semiconductor wafers which have large diameters, however, this represents a problem which has not as yet been satisfactorily solved.
FIG. 1 shows a semiconductor wafer which has an upper side 2 which is irradiated over its surface with electromagnetic radiation 3 of uniform intensity. Due to the heat emission 4 which occurs at an increased amount at the edges 5 of the semiconductor wafer 1, a lower temperature will result at the edges 5 of the semiconductor wafer 1 than that which exists in the center 6 of the semiconductor wafer 1.
FIG. 2 illustrates the curve of the intensity I.sub.M of the radiation at the center 6 of the semiconductor wafer 1 and the curve of the intensity I.sub.R of the radiation at the edges 5 of the semiconductor wafer 1 and these are identical in this case. FIG. 2 also shows the curve of the temperature T.sub.M at the center 6 of the semiconductor wafer 1 and the temperature T.sub.R at the edges 5 of the semiconductor wafer during a tempering or annealing process which has a heating-up phase t.sub.1 and a tempering or annealing phase t.sub.2. It may be seen from the curves of FIG. 2 that a uniform intensity distribution of the irradiation over the semiconductor wafer results in a non-uniform temperature distribution because due to the additional area at the edges 5 of the semiconductor wafer, the radiation of heat at the edges increases over the radiation at the center.
The prior art will also be discussed with reference to FIGS. 3 and 4.
FIG. 3 shows a semiconductor wafer 1 in which the edge regions 5 are irradiated with increased intensity relative to the center 6 of the semiconductor wafer 1. The intensity distribution of the irradiation is schematically indicated by arrows which differ in width to show this variation. Methods based on this principle are employed for improving the temperature distribution in the known rapid thermal processing systems, as for example discussed in the publications of Varian GmbH, D-7000 Stuttgart-Vaihingen, Brochure No. VAC 6057 and of the Peak Systems Incorporated, Fremont, USA, ALP 6000, of May 1986. The relationship of the intensity I.sub.M at the center 6 of the semiconductor wafer 1 to the intensity I.sub.R at the edges 5 of the semiconductor wafer is maintained constant during the tempering or annealing phase t.sub.2.
FIG. 4 shows the curve of the intensity I.sub.M of the irradiation at the center 6 of the semiconductor wafer 1 and the intensity I.sub.R of the irradiation at the edges 5 of the semiconductor wafer, and also shows the curve of the temperature T.sub.M in the center 6 of the semiconductor wafer and the temperature T.sub.R at the edges 5 of the semiconductor wafer during a tempering or annealing process having a heating-up phase t.sub.1 and a tempering or annealing phase t.sub.2.
The irradiation power is controlled by using pyrometer measurements in the center 6 of the semiconductor wafer so that a prescribed temperature-time cycle T.sub.M (t) is maintained in the center 6 of the wafer. During the heating-up phase t.sub.1, radiation is carried out with extremely high intensities I.sub.M and I.sub.R, and the edges 5 of the semiconductor wafer are irradiated by more energy than the center and are more rapidly heated.
When the center 6 of the semiconductor wafer has reached the desired temperature T.sub.S, the temperature at the edges 5 of the semiconductor wafer are noticeably higher because of the higher energy which has been applied to the edges. A uniform temperature distribution at the rated value T.sub.S is established after some time delay during the tempering phase t.sub.2 due to the constant intensity relationship I.sub.M /I.sub.R. During the cooling phase t.sub.3 after the irradiation is shut off, the edges 5 of the semiconductor wafer then cool more rapidly than the center 6 of the semiconductor wafer.
As a consequence of this dynamic behavior, the temperature-time curve T.sub.R (t) at the edges 5 of the semiconductor wafer thus differ from the temperature-time curve T.sub.M (t) at the center 6 of the semiconductor wafer. The time until the uniform temperature distribution in the stationary tempering phase has been reached is dependent on the mass of the semiconductor wafer 1 and upon the value of the rated temperature T.sub.S. For example, it is 5 to 10 seconds for a semiconductor wafer which has a diameter of 6 inches and for a rated temperature T.sub.S of 1100.degree. C. With short tempering times of, for example, 5 seconds, regions at the edges 5 of the semiconductor wafer can have a temperature which is up to 50.degree. C. higher on the average than regions in the center 6 of the semiconductor wafer. This difference becomes even larger for shorter tempering times.
In the prior art methods the inherent advantage of rapid thermal processing rapid heat-up rates and short dwell times at high temperatures, cannot be fully exploited because of the temperature non-uniformity which limits the yield. This has an unfavorable effect specifically on large semiconductor wafers with diameters of 6 inches or more that are common in the production of electronic components which are fabricated on a silicon base.
The temperature non-uniformity caused by the dynamic behavior is not solved by any known rapid thermal processing methods of the prior art.